Physics Help Forum diving board torque

 Kinematics and Dynamics Kinematics and Dynamics Physics Help Forum

 Jul 29th 2017, 09:43 PM #1 Junior Member   Join Date: May 2017 Location: Sri Lanka Posts: 19 diving board torque See this image There are 4 forces which are forming which are F1 downwards,(The force which is adding by the bolt) F2 upwards, and the mg and Mg that ate the forces adding by the man and the board. So those forces are taking to calculate the torque of it. But there is a force which is adding by yellow colour rectangle wood to upwards to the board. But I can't understand why that force is not inholve for the calculation. Attached Thumbnails
 Jul 30th 2017, 07:46 AM #2 Senior Member     Join Date: Aug 2008 Posts: 110 $\sum \tau$ about the left end where force $F_1$ is acting ... $\tau_{net} = mg(d_1+d_2+d_3) + Mg(d_2+d_3)-F_2(d_3) + \color{red} {F_1(0)}$
Jul 31st 2017, 07:12 AM   #3
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Location: Naperville, IL USA
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 Originally Posted by osalselaka But there is a force which is adding by yellow colour rectangle wood to upwards to the board. But I can't understand why that force is not inholve for the calculation.
There is only 1 force at F1 acting on the board, and it is due to the bolt that connects the board to the yellow rectangle block. That force either acts upward or downward - in the picture the author chose to show it acting down on the board, because it's pretty clear that if that force wasn't there the left end of the board would tilt upward. Hence a downward force must be exerted by the bolt on the board to keep it in place. And hence any supporting force between the yellow rectangle directly on the board (i.e. not through the connecting bolt) must be zero.

 Jul 31st 2017, 03:20 PM #4 Senior Member   Join Date: Apr 2015 Location: Somerset, England Posts: 688 I have no confidence in any author of the image you posted, even for example calculation purposes. The only thing I can see is likelihood of a lawsuit for manslaughter if anyone uses that diving board.
Aug 2nd 2017, 03:22 AM   #5
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Join Date: May 2017
Location: Sri Lanka
Posts: 19

 Originally Posted by ChipB There is only 1 force at F1 acting on the board, and it is due to the bolt that connects the board to the yellow rectangle block. That force either acts upward or downward - in the picture the author chose to show it acting down on the board, because it's pretty clear that if that force wasn't there the left end of the board would tilt upward. Hence a downward force must be exerted by the bolt on the board to keep it in place. And hence any supporting force between the yellow rectangle directly on the board (i.e. not through the connecting bolt) must be zero.
According to this image this image yellow colour block , bolt and the diving board are connect to each. Therefore why is there not a reaction force acting by the yellow colour block upwards? Why only by the bolt?

Last edited by osalselaka; Aug 2nd 2017 at 03:28 AM. Reason: spellings

 Aug 2nd 2017, 05:07 AM #6 Physics Team     Join Date: Jun 2010 Location: Naperville, IL USA Posts: 2,271 Assume for a moment that yellow block is massless. Every action has an equal and opposite reaction, so .. with the weight of the man at the right end of the board the board pulls upward on the bolt at the left end, and the bolt pushes downward on the board (that's F1). The bolt pulls upward on the block, and the block pulls downward on the bolt. The block pulls upward on the ground, and the ground pulls downward on the block (which means that the block needs to be secured to the ground). For both the bolt and block the two forces acting on them have magnitude F1 and are acting in opposite directions, so that the sum of forces on the bolt, and on the block, is zero (otherwise the system would be unstable). Notice that in this chain of forces the block does not act directly on the board - only the bolt acts on the board at the left end. osalselaka likes this. Last edited by ChipB; Aug 2nd 2017 at 05:11 AM.

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